1. Field of the invention
The present invention relates, in general, to a method for fabricating an electroluminescence device and, more particularly, to a method for the fabrication electroluminescence device employing a sputtering process to deposit a ferroelectric insulation layer on a transparent lower electrode of indium tin oxide grounded through an electrically connected limiter, preventive of darkening phenomenon of the transparent lower electrode.
2. Description of the Prior Art
Until recently, there was extensively used a cathode ray tube (hereinafter referred to as "CRT") as a information display device. To date, as the information display device is strongly required to be more light, more solid and more flat, research and development for next generation information display devices has actively proceeded. As a result, an electroluminescence device, a liquid crystal display, a light emitting diode, and a plasma display panel were developed.
Among such information display devices, particularly, an electroluminescence device, an active solid display device in which hot electrons produced by high electric field may be interacted with a luminescent center to emit light, largely attracts scientific and commercial attention and its development is being watched with keen interest because it can have large display area and show superiority in luminance, color contrast and view angle.
Recently, in an electroluminescence device which is based on ZnS, respective luminances of yellow color and green color have been able to be obtained largely enough to put the etectroluminescence devices into practice but neither red color nor blue color has been able to be obtained enough.
To accomplish multicoloration yet high luminance in an electroluminescence device, active research and study has been directed to novel luminescent basic substances, for example, CaS and SrS luminescent phosphors, instead of the conventional basic substance, ZnS.
Meanwhile, a white light EL device, a newly developed device, in which double luminescent centers are added into alkali earth luminescent phosphor has been and continues to be actively researched. Since it is proved that the white light EL device is able to control three primary colors with a filter as well as be utilized as a monocolor display device, it significantly attracts attention.
Now, in order to better understand the background of the present invention, a description will be made for a conventional electroluminescence device and a method for fabricating the same, along with its problems.
Referring initially to FIG. 1, there is shown a structure of a conventional electroluminescence device. As shown in this figure, the conventional electroluminescence device has a substrate 1 on which a transparent lower electrode 2, a first insulation layer 3, a luminescent layer 4, a second insulation layer 5 and an upper electrode 6 are in sequence deposited between the lower and the upper electrodes 2, 6 formed with their respective predetermined patterns.
Following is of a fabrication method for the conventional electroluminescence device.
First, on a substrate 1, for example, a glass substrate, there is deposited with indium tin oxide (hereinafter referred to as "ITO") at a thickness of about 2,000 Angstrom which is then subjected to photolithography, to form a transparent lower electrode 2 of ITO having a predetermined pattern.
Subsequently, using a sputtering process, a material selected from a group consisting of, for example, Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, Ta.sub.2 O.sub.5, SiO.sub.2, SiON, SrTiO.sub.3, BaTiO.sub.3, PLZT and PbTiO.sub.3, is deposited in a thickness of about 3,000 to 5,000 Angstrom on the substrate 1 provided with the lower electrode 2, so as to form an first insulation layer 3.
For more detailed description of this sputtering process, reference is made to FIG. 2 which shows a sputtering apparatus for the conventional electroluminescence device. As shown in this figure, the sputtering apparatus for the conventional electroluminescence device comprises a vacuum chamber 11 in which a substrate holder 12 and a sputtering target holder 14 are provided at an upper portion and a lower portion, respectively. The sputtering target holder 14 secures a sputtering target 13. On the other hand, the substrate 1 is fixated at the substrate holder 12 in such a manner that the face of the substrate 1 carrying the lower electrode 2 is toward the sputtering target 13.
In this state, Ar, a process gas, and O.sub.2 and N.sub.2, reactive gases, are charged through an inlet (not shown) into the vacuum chamber 11. Application of a high electric field provided from an external power equipment (not shown) across the vacuum chamber, that is, to the substrate 1 and the sputtering target 13 produces plasma of the reactive gases, depositing the first insulation layer on the face of the substrate carrying the lower electrode 2.
Thereafter, using an electron beam evaporation or sputtering process, a ZnS, SrS or CaS-based luminescent layer 4 is deposited in a thickness of about 6,000 to 10,000 Angstrom on the first insulation layer 3, followed by deposition of a second insulation layer 5 in a thickness of about 3,000 to 5,000 Angstrom on the luminescent layer 4. The second insulation layer 5 is made of the same material as the first insulation layer 3.
Finally, a metal, for example, aluminum is deposited at a thickness of about 2,000 Angstrom on the second insulation layer 5 and then, is subjected to photolithography, to form an upper electrode 6 having a predetermined pattern.
In such conventional electroluminescence device fabricated, when both the lower electrode 2 and the upper electrode 6 are applied by an alternating current voltage which is large enough to generate a high electric field of 1 MV/cm or more thereacross, the electrons which are in an interface state between the first insulation layer 3 and the luminescent layer 4 and in an interface state between the second insulation layer 5 and the luminescent layer 4 are accelerated and thus transformed into so-called hot electrons with tunneling to a conduction band of the luminescent.
While a portion of the hot electrons impact upon a luminescent center, for example Mn.sup.2+ doped in ZnS which is a base substance of the luminescent layer 4, to excite the luminescent center, a portion of the hot electrons ionizes the base substance, coupling with holes. As a result, electron-hole pairs are produced.
For a luminescent mechanism in the luminescence device, the electrons excited to the conduction band by the hot electrons fall into a valence band. In the meanwhile, a light corresponding to the same energy that difference between the conduction band and the valence band is emitted from the luminescent layer 4 through the substrate 1 to the outside of the electroluminescence device.
A significant disadvantage of the conventional fabrication method for luminescence device is that, when a ferroelectric material, for example, SrTiO.sub.3, BaTiO.sub.3, PLZT, or PbTiO.sub.3, is deposited to form the first insulation layer 3, an electric potential arises on a surface of the ferroelectric first insulation layer 3, leading to darkening the transparent lower electrode 2. As a result, the quantity of the light emitted toward the outside comes to be reduced owing to this darkening phenomenon.